Rock grinding system



June 18, 1963 P. A. H. H. FAHLSTROM ETAL 3,094,289

ROCK GRINDING SYSTEM Filed" Jan. 7, 1960 5 Sheets-Sheet 2 HENRY LENNRRTLUNDBEEG &-; Go2nu \navaz HoLMBEze XM mm m m ATTORNEYS P. A. H. H.FAHLSTROM ETAL 3,094,289

ROCK GRINDING SYSTEM 5 Sheets-Sheet 3 INVENTOR? -Gokm lne-vmz HOLMBERGBYW M ATTORNEYS June 18, 1963 Filed Jan. 7, 1960llllllllllllllllllllllllllllllllllllllll I! 5 Sheets-Sheet 4 vdn v &

ROCK GRINDING SYSTEM 4 Pr A. H. H. FAHLSTRCM ETAL June 18,1963

Filed Jan. 7. .1960

NIP d8 2. QOGIWQMDW 5 away? INVENT 0135 Pan. Auoelzs HERMAN HeunmassouFams'rkon HENRY Leuuag'r Luuoaeze -GOTZQN \usvmz Homeezci M M *MATTORNEYS June 18, 1963 P. A. H. H. FAHLSTROM ETAL 3,0 8

ROCK GRINDING s s'rw Filed Jan. 7, 1960 5 Sheets-Sheet 5 FEED RATE

CONTROL INVENTORS PnAnDRsHeRmn Hsunmessoufiumain Hiuzv Lmunm- LunbaufiE5602: .tucwmz. Romain;

ATTORNEY;

United States Patent 3,094,289 ROCK GRINDTNG SYSTEM Per Anders HermanHenningsson Fahlstrtim, Henry Lennart Lundberg, and Giiran IngvarHolmberg, all of Boliden, Sweden, assignors to Bolidens Gruvaktieholag,Skelleftehamn, Sweden, a joint stock company limited of Sweden FiledJan. 7, 1%0, Ser. No. 1,134 Claims priority, application Sweden (let.29, 1959 Claims. (Cl. 241-34) The present invention relates to a methodfor comminuting rock or other crystalline material in the presence ofwater to a substantially predetermined particle size and/or pulp densityand devices for carrying out the process. The invention especiallyrelates to such particle reduction processes in which the materialconsisting of an arbitrary mixture of coarse and fine pieces before itscomminuting is not subjected to any, or only little crushing, while theessential crushing and comminuting to the desired particle size iseffected in a grinding chamber, in which the material without foreigngrinding bodies present through the movement of the grinding chamber isbrought to act as crushing and grinding bodies and thus crush, grind andcomminute themselves.

Said method commonly called rock grinding, has obtained an increasedimportance for crushing and grinding ores in connection with theirconcentration, for grinding limestone, raw cement and other crystallinematerials. In the process the material to be crushed and ground, forinstance ore, which may contain pieces, the edges of which may have alength of up to 20 inches and more, is comminuted to a finely dividedproduct having the par- 'ticle size required for the commonconcentration processes. Owing to the fact that the material to beground is given a falling, rolling, or another mutual movement in thegrinding chamber, the different pieces of the material will crush,abrade and comm-inute each other. Thus, it is characteristic for themethod that foreign grinding bodies are not used, which is the case forinstance in rod and ball mills, whereas the crushing and grinding bodiesrequired for the reduction are continuously formed from the materialitself and are continuously substituted by new bodies. A further featureof the method is to charge the grinding chamber, a material beingsubstantially coarser than that which can suitably be ground in rod orball mills, whereby the equipment commonly used for the crushing of thematerial, for instance in jaw and cone crushers, totally or partly iseliminated from the treatment schedule.

It is also known to apply the principle for rock grinding to wetgrinding and in this case to carry out the grinding in about the sameway as in ball and rod mill grinding with the difference that instead ofballs there are used fine grinding bodies and instead of rods coarsegrinding bodies of a defined size by screening separated from thegrinding charge. Of said processes it is only the last mentioned whichhitherto has been more widely applied to in wet grinding, because sameis easily adaptable to present art. The first mentioned method which issimply called direct one step rock grinding has in spite of itstheoretically obvious advantages compared with other grinding systemshitherto obtained a restricted use, since said method has only beeneffectively carried out as dry grinding. The dry grinding method hasthen generally been dependent on the purpose of obtaining the product ina dry state for further treatment.

Direct one step rock grinding is nowadays effected in horizontal,rotating drum mills, the diameter of which generally is twice to fourtimes the cylinder length. The

3,094,289 Patented June 18, 1963 material to be ground is generally fedcontinuously to the mill which at its rotation imparts to the material amovement and brings about a comminution of the product. The size of theground product is adjusted by increasing or decreasing the rate infeeding the material to the mill. In the grinding operation it isdesired continuously from the grinding drum immediately to dischargefinely ground particles .as soon as they have formed. In the drygrinding type of mills this is carried out by means of a ventilationsystem connected to the mill, by which system a suitable air volume canbe forced through the mill in order to sweep away substantially allparticles below the desired particle size, as soon as said particleshave been freed. By the stream of air the particles are carried away toa collector, in which from the material of the desired particle size thecoarse, not finely ground particles are separated and re-cycled to themill. Although said system will secure a rapid exhaustion and separationof the fines or finished product, the system requires for its operationan essential additional power consumption. Furthermore, the equipmentrequired for the product circulation is extensive which results incomparatively high installation costs for dry grinding systems.

The possibility of using the principle of dry grinding of ore in onestep as a stage of its concentration, especially regarding the mostcommon concentration method, the flotation method, is further restrictedby the fact that grinding in the presence of water is an indispensablepreliminary step for the concentration. After dry grinding of an orewhich is then to be flotated, the finished product must be subjected toa special conditioning step with water and chemicals which processrequires additional installation costs as well as an essential powerconsumption for its operation. To the extent that dry grinding can beemployed at all the total efiiciency of the installation will thereforedecrease. In order to utilize the benefits of rock grinding in one stepin concentration of ore, an effective wet grinding method is thusrequired. The need for this is therefore constantly increasing. Theprocess of direct wet rock grinding of ore in one step having notearlier been commercially practised is due to the fact that severalconditions are to be met not only with respect to the mill itself butalso with respect to the other devices and their functions belonging tothe grinding schedule, which requirements must be fulfilled so that thisgrinding method compared with grinding in other types of mills andgrinding schedules shall give an improved result. These conditions whichhitherto have not been fulfilled in a satisfactory way are essentiallythe following:

The grinding is to be carried out with a higher efiiciency in water thanin air or in other words, fora certain size reduction in wet grindingthe power input must be possible to be made lower than for instance indry grinding;

Freed particles in the grinding chamber are continuously and immediatelyafter their forming to be discharged from the grinding chamber andseparated;

Thus, it must be possible in a controllable and reliable way tocirculate the discharged and partly finely ground product with a lowexpenditure of power;

The grinding shall be possible to carry out under adding ore at aconstant rate independently of arbitrary variations in the size of theore pieces;

The finely ground ore shall be obtained as a suspension with apredetermined particle size and/or density.

Although it has been evident some years ago that mills of about the sametype as used in dry grinding with advantage might be used in wetgrinding of ore in one step, it has not been possible to carry out saidwet grinding,

since not only the mill is essential, but the entire grinding scheduleand the way in which said grinding schedule can be controlled.

The present requirements of a wet grinding system are all fulfilled inthe present invention which relates to a process and devices forcarrying out the process preferably in direct rock grinding in one stepof ore together with water at a predetermined particle size and/ ordensity of the suspension formed thereof and its object is to provide aneifective, reliable and economical grinding schedule for Wet grinding.The process according to the invention comprises the steps of feedingunder addition of the water the product (ore) to be ground comprising anarbitrary mixture of coarse and fine pieces, to a grinding chamber atthe movement of which the product without foreign grinding bodiespresent is brought to act as crushing and grinding bodies and thereby tocrush and grind themselves, discharging from the grinding chamber amixture of water, fines and oversize and in one or more stepsclassifying or screening this product for separating the fines andreturning the oversize to the grinding chamber. According to theinvention the process comprises the steps of adjusting the addition ofwater to the mill by means of a density meter adapted to measure thedensity of the suspension discharged from the grinding chamber in such amanner that the suspension discharged from the mill obtains apredetermined particle size and density, respectively.

The method is furthermore characterized in feeding the ore or materialat a substantially constant volumetric rate into a rotating grindingdrum, the speed of which is continuously adjustable by changing the rpm.of the driving device, continuously measuring the quantity of solids inthe pulp discharged from the mill and the feed to the mill and comparingthe values obtained with each other, wherein deviations from apredetermined value are used to adjustably actuate the driving device ofthe mill in such a manner that the rpm. of the mill is changed in adirection to counteract said deviation.

The method is further characterized thereof, especially in grinding to aconsiderable fineness, that separation of notfinely ground material inthe suspension discharged from' the mill is carried out by separation inhydrocyclones in'two stages, said suspension being fed to the firstcyclone stage, from which the separated fine material is subjected torenewed separation in the second cyclone stage after an addition ofwater which, controlled by an apparatus measuring the density of thefinely ground pulp, separated from the cyclone, is adjusted in such away that said suspension obtains its predetermined particle size and/ordenity.

The process is finally characterized in discharging the suspension ofthe product totally or partly finely ground from the grinding drumthrough a scooping grate nad pumping it to a classifying device by meansof compressed air.

' According to the invention it has thus been possible ot reduce thepower consumption for the crushing and grinding by controlling thevolume of water added to the mill in relation to the feed in such amanner that the suspension discharged from the mill is given a densitywhich in dependence of the lump size and other properties of the feed isnot less than 0.5 and not more than 0.7 times the density of the orefeed. If this requirement is fulfilled, the power consumption forgrinding a product to a predetermined size will be lower by wet grindingthan by dry grinding, the case being the contrary, if the requirement isnot fulfilled. According to the invention it is therefore essential thatthe volume of water added to the mill is controlled by an apparatuscapable of measuring the density of the pulp discharged from the milland automatically adjusting the volume of water added to the mill, whenvariations of the feed are detected.

The inventors have further found that when new material is fed at aconstant rate to a grinding chamber operating in a closed circuit withclassifying devices which return coarse material, by measuring thequantity of returned material and calculating the ratio between saidquantity and the quantity of the ore fed to the mill per unit of timeand by comparing this value with a predetermined value, it is possibleto obtain information of the measures necessary to obtain a maximumoperating capacity of the grinding mill. Then it has proved suitable tomake said comparison automatically and accordingly to control the r.p.m.of the mill so that a certain predetermined quantity of the charge ofmaterial shall be circulated.

Furthermore, the inventors have found that classifying in two stages incyclones is more advantageous than classifying in either one stage incyclones or other classifiers or in two stages in other types ofapparatus alone or in combination with cyclones. It has now been foundthat one condition which is necessary for successfully effecting aclassifying in two stages is that the pulp is passed from the mill tothe first hydrocyclone for recycling to the mill of a decidedly coarsefraction and that the final separation of the fines or finished productis effected in the second cyclone after an automatically adjustableamount of additional water regulated by the desired density of thefinely ground pulp.

A still further object of the invention is that by carrying out theprocess special advantages can be obtained firstly if the grinding drumis provided with a scooping grate which permit rapid discharging of thefinely ground particles, the rods of the grates having a spacing whichpermits discharging the product with a particle size of up to 25 to 30ms, and secondly if the pulp discharged from the mill is transferred bymeans of an air-lift to the cyclone in such a manner that the product isdischarged by the airlift into a pressure tank mounted above the primarycyclone which is connected to the pressure tank by a pipe and that thefine material separated in the primary cyclone is passed to a secondcyclone by a centrifugal pump.

An embodiment according to the invention shall now be further describedwith reference to enclosed drawings showing a device for grinding in onestage of lead hearing sandstone ore having a specific gravity of 2.8with a largest lump size of about 400 mms. to a ground product, of whichpasses through a 200 mesh screen, the finished ore suspension .or pulphaving a dilution of 1.5 parts by weight of water on one part by weightof ore, which corresponds to a pulp density of 1.33.

FIGURE 1 is a schematic view of the apparatus for carrying the grindingprocess in accordance with the invention.

FIGURES 2, 3 and 4 are enlarged fragmentary schematic views showingportions of the apparatus with labels applied thereto.

FIGURES 5 and 6 are enlarged fragmentary sectional views showing indetail specific features of construction of the drum and the air lift.

FIGURE 7 is an enlarged fragmentary schematic view showing the detailsof the rock weighing device.

In the drawing a mill 1 is shown, consisting of a drum having asubstantially horizontal axisof rotation. The drum comprises acylindrical shell 2 and end closures 3, 4 which are preferably slightlyconical, so that the length of the drum increases towards its centre.The grinding drum is provided with two hollow trunnions 5, 6 by means ofwhich it is 'journalled in two plumber blocks 7, 8, each placed on itsfoundation 9. The driving mechanism of the mill comprises a gear rim 10attached to trunnion 6 outside the plumber block 8, and meshing twopinions 11 mechanically connected with the gear boxes 12, the inputshafts of which are in turn each connected with a driving motor 13. Themotors are preferably electric DC. motors the rotational speed of whichcanabe regulated continuously, for instance according to theWard-Leonard system. The Ward-Leonard system, which is a known method ofspeed control for large D.C.

motors, comprises a DC. motor (i.e. the motor the rotational speed ofwhich is to be regulated) the field winding of which is connected inseries to a DC. generator. The output of the generator is regulated bymeans of the field winding energizing current which is supplied from aseparate auxiliary generator, the armature of which is usuallymechanically coupled to the main generator. The two D.C. generators aredriven by a common AC. motor. The rotational speed control of the mainDC. motor is accomplished by varying the energizing current supplied tothe field winding of the main generator, said energizing current beingcontrolled by means of 2a rheostat controller connected in series in thecircuit. The system provides for a speed control of a very highefliciency within a wide range of rpm. and loads. Outside the gear thehollow trunnion 6 serving as an overflow tor the ground product has anextension ending in a funnel-shaped part 14.

The inner wall of the grinding drum is provided with a steel lining 15,16, 17 to prevent wear. The end closure at the outlet side of the drumis only partly lined, more specifically at the peripheral part so thatthe lining terminates in an annular edge surface. From said edge surfacean annular plate 18 with grates .19 mounted in a spaced relation to andparallel with the end closure 4 extends towards the centre and as acontinuation of the lining 15. In that way there is created a spacebetween the grate and the end closure, which space by means of a numberof radially extending partitions 20, serving as scooping members, isdivided into a corresponding number of sector-shaped chambers 21. Saidpartitions 20 also act as wear plates to protect the inner-wall of theend closure 4. Towards the centre the grate terminates in an outletfunnel 22 of solid material. The spacing of the rods of the grate isgenerally 8-Q0 mms. The ratio of shell length to diameter of thegrinding drum shown is about 1:3 (7 x 22 it), but according to theinvention the dimensions may vary within a wide range and are notcritical for the operation of the mill. Moreover, the power transmissionbetween the mill drum and the motor or motors may be effected in adiiierent way. According to the invention it is not either necessarythat the mill drum is journalled on horizontal trunnions but also millshaving vertical trunnions and being arranged for rotational or gyratorymovement may be employed. Into the hollow trunnion 5 serving as a feedopening issues a feed hopper 23. At the upper part of the feed hopper 23there is arranged the discharge roll of a conveyor belt 24 for feedingore to be comminuted from a storage bin 25. Between the bin 25 and theconveyor belt 24 there is mounted a weighing device 26. Below thetunnel-shaped part 14 of the hollow discharge trunnion 6 is arranged acollecting box 27, the bottom of which continues into a tube 28 arrangedwith a slope through the foundation 9. The tube 28 terminates in theinlet funnel 29 of an airlift. The air-lift consists of a downilow tube30 placed in a well 31 below the floor 32. At its lower end the downflowtube terminates in a frusto-conical part 33 having a circular bottomplate through a central bore of which there is inserted an air tubeadapted to be raised and lowered, a seal between the air tube 65 and thebottom plate 34 being effected by means of a gasket 36. The air tube 35terminates into a central upfiow tube 37 open at the bottom, mountedcoaxially of the downflow tube, the lower part of the tube ending at theapproximate level of the passage between the downilow tube 30 and thefunnel-shaped part 33. The upflow tube extends so far up- Wards that thepulp expelled through the upper opening 38 of the upflow tube by gravitycan be passed to subsequent treating stages. Above the upper end 38 ofthe upflow tube a splash plate 39 is arranged and a collecting funnel40, the outlet of which is connected to a pipe 41 which after a verticalstretch extends substantially horizontally and is connected to ahydrocyclcne 42. In the vertical part of the pipe 41 a pulp densitymeter 43 of a 6 known type is arranged, suitably based on the principleof gamma radiation. The pulp density measured by this indicator being afunction of the percentage of solids and water, respectively, of thepulp, is continuously recorded by a recording member 44. Said member isconnected to a regulator 45 which is connected to a control valve 46 foradjustable adding of water into the feed hopper 23 of the mill. On thehorizontal part of the pressure pipe 41 a flowmeter 47 is arranged, forinstance formed as a venturi pipe, the differential in pressure of whichis indicated by means of a recording device 48. The recording device 48for the flow measuring and the recording member 44 fior measuring thepulp density are connected to a calculating and recording device 49which is adapted from the incoming signals from the recording members 48and 44 to calculate and record the quantity of solids per unit of timepassing through the pipe 41. A regulator 50 is connected to thecalculating and recording member 49, said regulator controlling anadjustable valve 51 arranged at the delivery spout of the hydrocyclone42. The opening or the valve is continuously adjustable by a signal fromthe regulator Sit. The calculating member 49 is further connected to acalculating member 52 which moreover receives a signal from a gaugevalue transformer of the weighing device 26. The calculating member 52is capable of determining the quotient between the quantity of solidsdischarged from the mill and the quantity of new ore fed to the mill(the so-called circulating load). The calculating member 52 is connectedto a regulator 53 in the Ward-Leonard circuit of the drive motor(motors). Said regulator 53 is provided with such adjustable timedelaycircuits that momentary variations of the measuring signals do notinfluence the speed control of the mill but only prolonged changes inthe circulating load. The hydrccyclone 42 in its upper end has an outlet54 connected to a pipe 55 which ends into a pump well 56. Thehydrocyclone 42 is arranged in such way that the coarse fraction fedthrough the valve 51 can gravitationally flow into the feed hopper 23.The pump well 56 is by means of a supply pipe 57 connected to a pump 58which may for instance by a centrifugal pump of known type preferablydriven by a variable rotational speed motor 59, preferably a DC. motor,the rpm. of which is controlled according to the Ward-Leonard system.The power of the motor 59 and its revolution are suitably recorded bymeans of devices known per se which are therefore not to be furtherdescribed. In the pump well 56 a level sensing means 60 is arranged tosense the level of the pulp in the pump well. The level sensing means 60is connected to a control member 61 which is in turn connected to aspeed control 62 in the Ward-Leonard system of the pump motor 59. Fromthe pump 58 a pressure pipe 63 extends to the jetted tangential inlet ofa second hydrccyclone 64. A pulp density meter 65 with recording member66 as well as a flow member 67 of the same type as the flow meter 47 arearranged in said pressure pipe. The recording member 66 of the pulpdensity meter 65 and the recording member 68 of the flow meter 67 areconnected to a calculating member 69 adapted to calculate and recordfrom the incoming signals from the recording members 66 and 68 thequantity of solid per unit of time passing through the pipe 63. To thecalculating and recording member 69 is engaged a regulator part 70governing an adjustable valve 71 arranged at the bottom spout of thehydrocyclone 64, the opening of the valve being continuously adjustableby an impulse from the regulator 70. The hydrocyclone is arranged insuch a manner that the coarse fraction discharged through the valve 71may gravitationally fall into the inlet funnel 23. The hydrocyclone 64is at its upper end provided with outlet 72 serving to discharge thefinely ground products, said outlet 72 being connected to a pipe 73. Inthe pipe 73 is connected a pulp density meter 74 of essentially the sametype as the meters 43 and 65. The pulp density meter 74 is connectedwith a recording instrument 75 and this instrument in turn to aregulator 76, which is engaged to a control valve 77 for water placed ina pipe 78, terminating in the pump well 56. At the opening of the pipe73 a sampler 79 of the type described in my copending application SerialNo. 43,381, filed July 18, 1960 is suitably arranged. After the saidsampler 79 there are conventional devices 80 for the ore dressing, forinstance by flotation. Said devices are provided with members fordischarging concentrate 81, for tailings 82 and for returning themiddlings to the grinding circuit 83.

The air pipe 35 of the air-lift is by an air pipe 84 connected with acompressed air tank 85 which by a pipe 87 is connected with a compressor86. 'In the inlet funnel 29 of the lair-lift is mounted a level sensingmeans 88 arranged continuously to sense the pulp level in the downflowpipe 30. The level sensing means 88 is connected to a control member 89which is connected to a control valve 90 of the pipe 84.

Said system comprises also feeding devices for water. These devices forinstance include a water tank 91 or pressure pipe arranged in such amanner that via the pipe 92 water is passed to the feed hopper 23. Thepipe 92 is provided with the aforesaid control valve 46 which is engagedto the control member 45 of the pulp density meter 43, 44, and said pipe92 is moreover suitably provided with a water flow meter 93. From thewater tank 91 also emanates the above mentioned pipe 78 provided withcontrol valve 77, said pipe 78 terminating in the pump well 56. A waterflow meter 95 is connected to the pipe 78. Furthermore, a pipe 96 for adiluted suspension of finely ground ore from the ore dressing devices 80and their devices for recycling the return pro-ducts 83 terminate in thepump well 56. The water tank 91 is connected with the dressing devices,said pipe 94 being provided with a control valve 97 and a water flowmeter 98.

The mill operates as follows:

By the conveyor belt 24 is fed to the mill 1 from bins either ore whichhas been screened in dilferent fractions and then again mixed in acertain ratio or unscreened ore at an approximately constant feed rate,for instance 20 metric tons per tour, which rate is adjusted to adesired value and recorded in the weighing device 26. In this operationthe ore has been subjected only to a minor crushing action to break thechunks. The mill 1 is rotated through the motors 13 via the gear boxes12 and pinions 11 meshing the gear rim 16. The r.p.m. of the mill hasnow been adjusted in such a manner that it is adapted to the size of theore feed as well as to the degree of grinding desired, which in thedescribed case corresponds to an r.p.m. of 70% of the critical speed,that is the highest rotary velocity, at which the grinding drum mayrotate without the grinding product by the centrifugal force followingthe drum in its rotation. If the mill is arranged for operation belowthe critical speed, the rpm. of the drum usually is within the range of50-100% of the critical speed, whereas if the mill is normally intendedto operate at an r.p.m. over the critical speed in order to reduce thewear the r.p.m. is adjusted somewhere between 100% and 150% of thecritical speed. The ore falls through the feed hopper 23 via the hollowtrunnion into the grinding chamber and is set in motion. Simultaneouslywater is supplied in a controllable volume through the pipe 92 by meansof the valve 46. In addition, the oversize material separated in thehydrocyclones 42 and 64 in the form of an aqueous suspension is fed tothe feed hopper 23. Owing to the rotation of the mill drum the ore isrolled and is crushing, abrading and comminutingitself. Furthermore, bythe motion of the mill acontinuous discharge of material takes placethrough the grate 18 to the scooping chambers 21, in which chambers thematerial by the rotation of the mill is lifted and then falls throughthe open space 19 between the outlet funnel 22 and the dischargechambers 21 and therefrom to the discharge trunnion 6. It flows like aslurry through the opening 14 of the mill trunnion. Depending on theinterspace of the grate which generally varies between 8 and 20 mms. thedischarged, partly ground product contains particles of up to 25-30 mms.edge size. The mixture of material and water flowing from the milltrunnion falls into the funnel-shaped collecting box 27 and thereafterflows by gravity through the pipe 28 to the inlet funnel 29 of theair-lift. The airlift operates in such a manner that air is continuouslyled from the air pressure tank which in turn is fed from the compressor86 over the pipe 87. The compressor has a larger capacity than thatrequired for the volume of air normally taken from the pipe 84, andtherefore the compressor is provided with relief devices in a knownmanner. Air is blown in through the air tube 35 of the upflow tube 37 ofthe air lift. The suspension of ore and water conveyed to the inletfunnel 29 of the air lift falls through the downflow tube 30 at the sametime as the pulp is caused to flow upwardly in the upfiow tube 37 underthe influence of the difference between the hydrostatic pressureprevailing in the downflow tube 30 and the hydrostatic pressure of themixture of suspension and air prevailing in the upflow tube. To have theair lift to operate at maximum efficiency which is essential for theeconomy of the pumping operation, the inlet pressure of the air and theair volume must be adjusted in such a way that the pressure is justsufiicient for the operation of the pump. In the present case this isachieved by means of the aforesaid level sensing means 88 arranged inthe inlet funnel 29 of the downflow tube 30, which sensing means 88 iscontinuously sensing the pulp level in the downflow tube and via thecontrol member 89 controlling the volume of air through the valve 90 insuch a way that a constant fluid level is obtained in the downflow tube.If the flow of the suspension discharged from the mill is increasing ata certain setting of the air volume this results in that the pulp levelin the downflow tube tends to rise, which is measured by the levelsensing means 88 giving a signal to open the valve 90 by means of thecontrol member 89, so that larger air volume is released to the air pipe35, which results in an increased feed through the air lift.Simultaneously the tapping of air from the air pressure tank 85 isincreased which in turn results in an increased delivery of air from thecompressor 86. A decrease of the discharge of pulp from the mill at acertain setting of the air volume through the air pipe 35 results in thereverse controlling procedure.

The mixture of suspension and air fed through the upflow tube 37 isdeflected at its outlet from the same by the splash plate 39 of thecollecting funnel 40, the air being separated and removed while thesuspension falls down into the collecting funnel 40 and by gravity flowsto the cyclone 42 via the pressure pipe 41. In the cyclone 42 aseparation between the coarse and fine material discharged from the millis effected, the coarse fraction being discharged through the adjustablevalve 51 in. the apex of the hydrocyclone and thereafter falling intothe feed hopper 23, and the fine fraction is conveyed through the topoutlet 54 of the cyclone and is subjected to a further classifying. Thecyclone 42 has been made with such dimensions that it is capable ofseparating substantially all material having a particle size 1 mm. as acoarse fraction and a pulp density higher than that of the pulpdischarged from the mill, while material having a particle size 1 mm. isdischarged through the topoutlet 54. For the operation of the cyclonethere is required a hydraulic head or pressure of about 5 to 8 meterswhich is achieved due to the fact that the collecting funnel 441 isarranged at corresponding height over the opening of the inlet pipe 41of the cyclone. The pulp density meter 43 continuously measures thedensity of the pulp passing through the pipe 41. If the pulp densitymeter is of the gamma radiation type, the measuring is based on theabsorption of the radioactive radiation passing through the pipe 41. Theabsorption increases with increasing specific gravity of the suspensionin the pipe 41. The measured value of the pulp density is continuouslyrecorded by the recording instrument 44. Owing to the fact that the pulpdensity meter 43 is mounted on a vertical downfiow pipe the incorrectmeasurings caused by strata in the pipe are negligible which isessential for a good control. The recording member 44 of the pulpdensity meter is connected to the regulator 45 which is in turnconnected to and governs the water valve 46 controlling the feed ofclear water to the mill. The desired value of the density of the pulpdischarged from the mill is adjusted on the regulator 45, said densitybeing according to the invention Within the range of 0.5 to 0.7 timesthe density of the solid ore. In the example described relating togrinding lead ore having a density of 2.8 it is most suitable aiming ata pulp density of 0.63 times the density of the ore, that is 1.75. Thus,if the volume of water fed to the mill decreases in relation to the orefeed, the percentage of solids in the suspension discharged from themill will increase which causes an increase of the density of thesuspension. Said increase of the density will immediately be recorded bythe instrument 44, which via the regulator 45 emits a signal to increasethe opening of the valve 46, the water supply being increased and thedensity of the suspension discharged from the mill being reduced towardsthe desired value, until same is achieved. If, on the other hand, thewater supply in relation to the ore feed should have increased, thedensity of the suspension discharged from the mill will decreaseinstead, which is sensed in a corresponding way and causes the reversecontrolling procedure. This characteristic of the grinding system isvery essential, since even short variations of the density of thesuspension adversely affect the grinding capacity of the mill. For atsuch variations of density the system tends to lose its balance which isdifficult to restore. This is perhaps best explained by the fact thatthe quantity of solids in the suspension discharged from the mill at thedescribed reduction ratio for said ore is generally to times thequantity of fresh ore continuously fed to the mill. Thus, in order tohave the system operating satisfactorily it is required that the densityof the suspension can be rapidly measured and without any substantialerrors which is satisfactorily described in the schedule.

On its passage through the pressure pipe 41 to the hydrocyclone 42 thesuspension passes through the flow meter 47 formed as a venturi pipe, apressure head being created which is a function of the volume per unitof time and the density of the suspension flowing through the pipe. Thispressure head is recorded in the member 48. From said member a signal istaken which is a function of said pressure head, and furthermore anelectric signal is taken from the pulp density meter 44, the strength ofthe signal being proportional to the pulp density. These signals aretransmitted to the calculating member 49 which by means of an analogcomputer from the incoming signals is capable of calculating thequantity of solids per unit of time through the pipe 41 as well asrecording the value obtained. Thus, in the calculating and recordingmember 49 the quantity of solids continuously discharged from the mill 1is indicated, for instance in metric tons per hour. The regulator 50engaged to the member 49 is capable of adjusting the valve 51 of thehydrocyclone 4-2. This control procedure implies that said control valveis constantly caused to assume a size having a definite proportion tothe quantity of solids per unit of time fed through the pipe 41 to thehydrocyclone 42 and in the present case in such a way that the controlvalve 51 is opened if the quantity of solids increases, whereas if thequantity of solids decreases the valve is closed. In this way acontinuous adjustment of the desired ratio of separation of the cycloneis obtained, independent of variations of the incoming quantity ofsolids. If this quantity increases, the cyclone tends at a constant,unchanged bottom opening to give. a coarser fine inaction, whereas ifthe quantity of solids to the cyclone decreases, there will be obtaineda finer fine fraction. According to said control system these variationsmay be totally eliminated. This results in that an accurate classifyingin the second cyclone stage can be obtained. The value of the quantityof solids discharged from the recorded in the recording member 49 isalso used to control the rpm. of the mill. This method will be furtherdescribed hereinbelow. An essential condition for the classifying to becarried out in the hydrocyclone 42 at a good separation sharpness withrespect to the fact that the material fed to the cyclone has a particlesize from 25 to 30 mms. down to a few ,u, is that the classifying takesplace at a high pulp density. This is achieved by the described pump anddelivery systems into which no foreign water will pass in anincontrollable manner which is generally the case when using centrifugalpumps for the pumping.

The suspension essentially freed from 1 mm. particles is continuouslyfed through the outlet 54 and passes through the pipe 55 to the pumpwell 56. From the pump well 56 the suspension is sucked into the pump58. In order that the pump 58 at every moment shall carry away the samequantity as is fed to the pump well 56, the rotary speed of the pump isadjustable. The impulse to the right adjustment of the number ofrevolutions is obtained from the level sensing means 60; which iscontinuously sensing the pulp level in the pump Well. The regulatorcooperating with the level sensing means 60 continuously emits a signalto the speed control 62 of the Ward-Leonard system of the pump motor 59.The device operates in such a way that if, at a certain state ofequilibrium between the rpm. of the pump and the supplied volume ofpulp, the latter for instance decreases and, consequently, the level inthe pump Well 56 is lowered, this process is immediately detected by thelevel sensing means 60 which through the control member 61 emits asignal to the speed control 62 to reduce the r.p.m. of the pump 5S. Inthis way the capacity of the pump 58 is reduced and the level tends torise in the pump well 56. The regulating operation is in principle thesame but reversed, if the quantity supplied to the pump well 56 isincreased. Therefore, the system is characterized in that the supply tothe pump well 56 is at all times in equilibrium with the withdrawaltherefrom which renders the pump 58 to operate at the highest possibleefficiency independent of variations of the incoming volume. The purposeof the pumping is to force the suspension through the hydrocyclone 64.In said hydrocyclone the final separation between the finely ground andnot finely ground particles is carried out of the material dischargedfrom the mill 1 and which has been subjected to a first classifying inthe cyclone 42. The second cyclone which does not need to treat thewhole quantity of the suspensions discharged from the mill, is therebymade smaller than the first cyclone but requires a higher feed pressureowing to the separation being carried out at a finer grain size.

On its passage through the pressure pipe 63 to the hydrocyclone 64 thesuspension flows through the measuring member 67, formed as a venturitube, a pressure head being created which is a function of the flow rateand the density of the suspension. This pressure head is recorded in themember 68. From this member is taken a signal, the strength of whichbeing a function of said pressure head, and furthermore a signal istaken from the pulp density meter 65, the strength of which isproportional to the density of the suspension. These signals aretransmitted to the calculating member 69 which through an analogcomputer from the input signals is capable of calculating and recordingthe quantity of solids passing through the pipe 63 per unit of time.

Thus, in the calculating and recording member 69 the quantity of solidscontinuously fed to the hydrocyclone 64 is indicated, for instance inmetric tons per hour. The regulator 70 engaged to the member 69 iscapable of adjusting the bottom opening 71 of the hydrocyclone. Thiscontrol procedure implies that said bottom opening 71 is constantlycaused to assume a size having a definite relation to the quantity ofsolids fed through the pipe 63 to the hydrocyclone 64 per unit of time.The operation of the cyclone has the effect that the ratio of separationof the cyclone and in this case the particle size of the fine fractionpassing through the top outlet 72 of the cyclone 64 attains apredetermined value, independent of variations of the quantity ofmaterial fed to the cyclone. Furthermore, a coarse fraction having adensity higher than the suspension is discharged from the mill throughthe bottom valve of the cyclone.

When, for instance, the quantity of solids per unit of time of the finefraction discharged from the hydrocyclone 42 and pumped to thehydrocyclone 64 increases, this increase is immediately sensed by themember 69, the regulator 70 adjusting the bottom valve of the cyclone toincrease the opening thereof. This results in that a larger quantity ofmaterial will be returned to the mill 1 at the same time as the desiredparticle size of the fine fraction is maintained.

If, on the contrary, the quantity of solids per unit of time of the finefraction discharged from the hydrocyclone 42 decreases, the controlprocedure will be reversed so that it is constantly achieved that theground product obtains its predetermined particle size. To secure thiscondition it has been found that the described schedule with classifyingin two stages is especially appropriate.

In the first cyclone 42 which is relatively insusceptible to variationsof the particle size of the incoming material, all coarse particles areseparated and the fine fraction is given a particle size distributionwhich is practically constant during the course of time and the quantityof which may vary, whereas in the second cyclone 64 the particle sizeand density of the ground product is finally adjusted at the desiredvalue. The separation limits of both cyclones are in this case adjustedin such a manner that they each operate at the highest possibleclassifying efficiency which is among other things achieved by thedensity of the suspension in the first cyclone stage being high, whereasin the second stage the density lowered on account of further additionof water.

By the described control of the bottom opening the coarse fractionsre-cycled to the mill from the cyclones 42, 64 are given a density whichsomewhat exceeds the density of the suspension discharged from the mill,and also in this respect the system is automatic, if the opening of thecyclone has been chosen in a suitable manner in relation to the normalvariations in the circulating load as well as to the desired separtionefliciency of both cyclones.

The finely ground material leaves the cyclone at the top through theopening 72 and has then obtained the desired particle size, in this case80% below 200 mesh. Therefore, it is withdrawn from the grinding systemthrough the pipe 73. For the subsequent process, for instance aflotation, the ratio of water to solids of the pulp which is a functionof the pulp density should be kept within narrow limits. To meet thisdemand the density of the suspension of finely ground ore coming fromthe cyclone is continuously measured in the pulp density meter 74,attached to the pipe 73, and the density is recorded by the instrument75. The desired value of the density of the suspension is adjusted onregulator part 76 belonging to this instrument, in this case 1.33. Withthese devices the pulp density of the discharged, finely ground productis continuously measured, a deviation from the desired value causing acontrol measure. For the regulator 76 is engaged to the water controlvalve 77 of the pipe 78 terminating in the pump well 56. If the finelyground fraction discharged from the cyclone through the aperture 72 isundergoing a change in such a manner that the quantity of solidsincrease in relation to the volume of water, the density of thesuspension increases,

which is immediately sensed by the pulp density meter 74 and recorded inthe instrument 75 as well as transmitted to the regulator 76 governingthe control valve 77 to increase the aperture so that the supply ofwater to the pump well 56 increases. This will continue until the valuemeasured by the pulp density meter 74 coincides with the desired valueof the regulator 76. The regulation procedure is reversed, if the fineor finished fraction discharged from the cyclone through the aperture 72obtains a lower solids to water ratio than in normal setting. Thecontrol measure will then be the reverse of the described one.

There are several reasons for the dilution or the density of the pulpdischarged from the cyclone 64 through the aperture 72 being varied. Thedensity of said suspension is determined by the density of thefine-granular suspension discharged from the hydrocyclone 42 through theaperture 54 as well as by the separation rate in the hydrocyclone 64,which is determined by the setting of the bottom aperture in relation tothe quantity of material fed. The density of the finely ground productis furthermore determined by the density of the quantity of the returnmaterial which is passed through the pipe 96 to the grinding circuitinto the pump well 56. From the concentration circuits a varyingquantity of return material is normally re-cycled, the return materialconsisting of a mixture of water and partly concentrated ore whichrequires renewed treatment in grinding as well as flotation. Said returnmaterial is discharged in the concentration circuit 80 through thedevice 33 and is conveyed to the pump well 56 through the pipe 96. Thequantity of the material as well as the volume of water of said returnmaterial varies within broad limits depending on the nature of the oreand the way in which the flotation process itself is carried out. Thewater following the return material is used as dilution water in thesuspension at its treatment in the hydrocyclone 64. In the fineclassifying which is desired there is obtained an improved separation,if water is added to the suspension, as stated above. Instead of addingonly clear water, in this way already utilized water is used in theprocess. The volume of water following the return material through thepipe 96 is, however, generally not sufficient for the suspension offinely ground material discharged from the cyclone to obtain asufficiently high percentage of water, and therefore it is necessaryalso to supply the pump well 56 with clear water, the quantity of whichbeing automatically controlled by the valve 77 in the manner described.In order to carry out all these regulating functions in a satis factoryway, it is necessary that the capacity of the pump is adjustable so thatthe rate of withdrawal pulp by the pump is the same as the delivery tothe pump. This is noted to emphasize that the cyclone pump is made withvariable capacity, which is in the present case attained by the factthat the pump motor has been given a variable rotary speed.

At the outlet of the pipe 73 the suspension of finely ground particlesand water passes through the sampler 79 adapted continuously to samplingsmall quantities of the finely ground ore. Said sample can, if desired,be continuously classified, changes of the screen tests being recordedand used to control the rpm. of the mill (for instance according toUnited States patent application Ser. No. 748,514). This can be effectedin such a manner that the sample taken at a rate of for instance 10 kgs.per hour is continuously classified in a sieving machine with meshapertures adapted clearly to show the particle size of the groundproduct. In the present example a revolving screen having three wovensieves of 0.1, 0.06 and 0.044 mm. aperture is used. The weight of thefractions retained on the sieves are weighed continuously on recordingbalances, the value of the sieve analysis thus obtained being used tocontrol the r.pi.im. of the mill.

It has been described above how the regulation of the grinding productis continuously controlled in a grinding scheme to a predetermined sieveanalysis and predetermined degree of dilution, in which scheme there areimportant variations of quantities of material and volumes of water. Themethod described and the devices for the circulation and classifying ofthe ore in the hydrocyclones are especially suitable for saidregulation. Since the detention period in the cyclone devices used andthe auxiliary equipment is 2 to 4 minutes, a change in the operationconditions of the mill will be very rapidly manifested in thecomposition of the finely ground product. n the other hand, thistherefore necessitates effecting regulation functions automaticallysince in such a system the manual handling will be too time-consuming tohave the system operating in a satisfactorily manner. By the inventionthere have been achieved very simple automatic devices as well as lowoperating costs for the circulation of the material. The purpose of theregulation methods now described in the first hand is to equalize andneutralize the effect of minor changes of the composition of thefinished product especially in the circulating load and volume of Water.According to the invention a further factor, that is the r.p.m. of themill, which may be if desired governed by impulses based on thecirculating load can be introduced as a regulating factor to enable apredetermined sieve analysis of the ground product also at very strongvariations of the grinding properties of the ore.

In grinding of a certain type of ore with constant feeding of new ore tothe tumbling mill of a certain r.p.n1., for instance 70% of the criticalspeed, the grinding and screening system will automatically operate inbalance. In the mill a crushing and grinding charge of totally or partlydisconnected bodies of a certain composition is formed, which gives thematerial discharged from the mill a certain particle size in so far asthe density of the suspension discharged from the mill is kept withinthe range given. However, if the grinding properties of the ore arechanged, for instance if in the feeding the ore attains anothersize'distribution, density, fragility, etc., this will manifest itselfin a change of the circulating load.

The calculating member 52 continuously estimates the ratio between thequantity of solids discharged from the mill and the quantity of freshore fed to the mill (circulating load) from the signals emitted from thecalculating member 49 of the pulp flow meter 49 and the gage valuetransformer of the weighing device 26. At balance in the system saidmember 52 shows in the present case a value of the circulating loadwhich is for instance times the quantity of the ore fed per unit oftime. An increase of the quantity of the material discharged from themill is immediately measured and recorded in the calculating member 49.Initially this results in that the regulator 50 adjusts the bottomaperture 51 of the hydrocyclone 42, so that a larger quantity ofmaterial corresponding to the increase is returned to the mill 1. If theincrease is very sudden, the bottom valve 51 is not always capable ofdischarging the total additional quantity of the material, but a certainpart thereof is passed also to the hydrocyclone 64, the bottom aperture71 of which is also increased by regulation in a manner earlierdescribed, so that the finely ground product discharged from the cyclone64 is maintained at the predetermined composition. However, if theincrease is of considerable duration, for instance dependent on changedgrinding conditions in the mill 1, the quantity of the materialdischarged from the mill will successively increase according as anincreased quantityof material is re-cycled from the cyclones 42, 64,from which it is evident that the grinding conditions of the mill 1 havebeen changed, in this case towards lower grinding capacity. It ispossible to neutralize such a change by reducing the feed of new ore tothe mill 1. Such a reduction, however, is also causing a decrease of thefinely ground ore delivered from the system, and since the processfollowing the grinding for its most economical operation requires thatthe feed of ore is constant, this regulation method is less suitable.Instead of that the information and the changes of the quantity of thematerial discharged from the mill can be used to control the rpm. of themill. From the member 52 a signal is transmitted to the regulator 53 inthe Ward-Leonard system for the drive motor or motors 13 of the mill 1and then readjust the number of revolutions of the mill.

If the circulating load calculated in the member 52 increases inrelation to the adjusted desired value a signal from the regulator 53 istransmitted to the drive motor or motors 13 for adjusting of the rpm. ofthe mill according to a predetermined program. In the present examplethis program implies a reduction of the rpm. of the mill, if thecirculating load increases, and an increase of the rpm, if thecirculating load is reduced which among other things results in acertain time delay so that only changes of long duration but not ofshort duration are recorded in the circulating load, and also that theregulation of the r.p.m. is carried out in several smaller intervals.This results in a charge which is capable of finely reducing moreeffectively which in turn results in a reduced circulating load, so thatsaid charge approaches the desired value of the system and automaticallywill be restored to said value. The case is the reverse if thecirculating load decreases which shows that the grinding tends to becomefiner than desired. At a constant feed of ore this will be compensatedaccording to the program by an in crease of the rpm. by means of theregulator 53 and the drive motor 13.

According to the invention also other regulating programs than thedescribed one can be used. The program is then dictated by the crushingand grinding properties of the ore or material. The essential matter ofthe regulation operation described is that in combination with thedilution control and the classifying process earlier described, saidoperation enables a complete control of the reduction operation whichhas not been possible earlier.

Some further essential features of the invention will be pointed outbelow.

In wet rock grinding it is important that the finely ground particlesare immediately discharged from. the mill. This is most suitablyaccomplished, if the mill is provided with a scooping grate. It is alsodesirable to make these grates wider than in mills for scooping in ballor rod mill grinding. By this method it is rendered possible veryrapidly to discharge the material from the mill. However, this has theresult that the product discharged from the mill will contain bodies ofup to 20 to 30 mms. edge length. Furthermore, the suspension dischargedfrom the mill is relatively viscous on account of the consistency thepulp has within the optimal density range. The separation of the finelyground product and the re-oycling of not finely ground product to themill which in common ball or rod mill grinding does not result in anyspecial difiiculties, requires in rock grin-ding special devices. Forinstance in ball mill grinding the partly ground pulp discharged fromthe mill drum is permitted to flow by gravity to a mechanicalclassifier, in which coarse, not finely ground particles are left and bya raking mechanism or a screw feeder are scraped towards the feed end ofthe mill, and then said coarse fraction by means of for instance aconveyor or by a lifting scoop is transported to the mill. Said systemcannot be adapted in the present mills, since the diameter of the-millis too large to permit such an arrangement. In order to solve theproblem it has been eanlier tried either to place a classifier below themill and by means of a chain-bucket elevator to lift the coarse materialinto feed trunnion of the mill, or there has been chosen a method bymeans of a chain-bucket elevator to lift the teen quantity of thematerial and Water discharged from the mill to classifiers placed abovethe centre line of the mill wheel in such a way that the coarse materialfraction can gravitionally fall into the mill trunnion. As thecirculating load in rock grinding is often up to to times the quantityof the ore fed, the quantity of the material which shall be transportedby said chain-bucket elevator will be very large and also expensive withregard to installation and maintenance. Furthermore, a chainbucketelevator has the disadvantage that by certain relations between theparticle size of the solids and the percentage of water the materialWill sediment in the buckets, which results in said buckets totally orpartly being incapable to transport. It has not been possible by known,rotating centrifugal pumps successfully to pump the suspensiondischarged from the mill to a classifier situated above the mill, partlydue to the coarse particle size and partly due to the pulp density beingtoo high with respect to the general purpose of a. centrifugal pump.This has caused a substantial wear of the pump, irregular speed and,accordingly, unsatisfactory operation conditions. The use of an air liftto raise the suspension discharged from the mill, containing even 'verylarge ore particles at a low percentage of water, affords manyadvantages and makes it possible for the grinding scheme described tooperate effectively. A correctly dimensioned air lift is operating veryregularly and is capable at a constant efficiency to raise a pulp, thequantity of which varies for instance as 5:1. The operation of theairlift is not disturbed by relatively coarse bodies fed to the lift.The air lift does not contain any moving parts and shows aninsignificant wear and is therefore very reliable. Finally the air lift,in any case in the grinding of complex sulfide ores, has the advantagethat the ore in the grinding is subjected to a vigorous airing whichpromotes the subsequent flotation. The air lift is suitably installed ina well which is to be blasted in connection with the building of themill hall. In certain cases it is possible to simplify the constructionof the pump as far as the downflow tube not being placed in a well butdirectly as a hole in the ground. In this case the inlet air tube isplaced within the downflow tube and suitably at the wall of the tube andending through a bend of the upflow tube 37. In certain case it issuitable to arrange the air lift in two or several stages, each of whichraising the pulp half or less parts of the way. This has the advantagethat the immersion depth below the ground surface needs not to be large.In order that the air lift shall operate with the highest possibleefiiciency the compressed air shall not be compressed to a higherpressure than is required for the air just to be forced into the upfiowor raise tube. In the starting period and in certain instances it istherefore desirable to supply air of a pressure higher than is suitablyaccomplished by a separate compressor. For air lifts intended for millshaving diameters up to 10 metres, an air pressure of about 28 p.s.i.gage (2 kgs./sq. cm.) is required, if the air lifts are made in twostages. Thereby a level difference is obtained between the collectingfunnel and the hydrocyclone 42 of about 5 to 6 metres, which generallyis satisfactory for the operation of the hydrocyclone. The compressedair from piston compressors generally contains a minor quantity of oilwhich can adversely affect the subsequent flotation of the finely groundsuspension, and therefore an oil filter should be inserted between thecompressor 86 and the compressed air tank 35. An air cooler for thecompressed air is not generally necessary.

In the classification in hydrocyclones in two stages special advantagesare also attained. Due to the fact that the suspension discharged fromthe mill is classified in a hydrocyclone, in any case a primaryseparation of the coarse fraction, a very reliable and simple device forthe classification has been obtained. It is earlier known to usecyclones for classifying fine granular material. It

has now been rendered possible successfully to carry out also aclassification of a material which has been previously considered asbeing possible only to be separated in mechanical classifiers. In thepresent case a mechanical classifier should have the dimensions 4 x 10mm. which is now substituted by a cyclone of 600 mm. diameter. In finegrinding it is suitable to carry out a subsequent classification in afurther, somewhat smaller cyclone, since the coarsest fraction has beenwithdrawn and returned to the mill. This suspension is then totallyfreed from coarse particles which may have an abrading effect on arotating pump so that the pumping to the second hydrocyclone stage isadvantageously carried out by means of such type of pump. The fineseparation in the second cyclone generally requires a higher inletpressure to the cyclone in the second stage than in the first stage. Ifthe entire classifying operation should be carried out in one cyclone,there should be required for said cyclone an inlet pressure as high asfor the second cyclone which means that the total quantity of the pulpof higher density and containing very coarse material must be pumped ata higher pressure which would essentially reduce the economy of thepumping operation. Thus, in the present invention the pumping to theseparate hydrocyclone stages has been adjusted in such a manner thatthis part of the installation obtains the highest possible efficiency.

If the ore is not desired to be ground as finely as required forflotation which is the case for instance in iron ore dressing, it issuitable to carry the classification in only one step in thehydrocyclone 42. In this case it is suitable for the control of thedilution or density of the finely ground pulp discharged through the topopening 54 of the hydrocyclone, to mix the suspension with a variablevolume of water in a mixing vessel, said addition being controlled by apulp density meter which also can be made automatically adjustable inthe same manner as earlier described. However, it is also possible tomake this control of the volume of water in the air lift, if specialmeasures are taken to measure the added volume of clear water which thensuitably takes place in the inlet funnel 29 of the air lift. If theadded quantity of clear water is measured and recorded, the effect ofsaid quantity on the density of the suspension measured in the pipe 41can be compensated by a special calculating member. The measuringinstruments 44, 48 and 49 which measure, record and control the quantityof the material discharged from the mill are, however, not affected bythe addition of water in the air lift 29.

In grinding to especially coarse particle size of the finely groundproduct, other separating members than hydrocyclones, for instancescreens may be preferred and then such screens are substituted for saidcyclones 42 and 64. In this case the regulation according to theinvention is facilitated to continuously measuring the density of thematerial in the pumping and measuring of the circulating load, theoperation of the mill being controlled in a similar way as earlierdescribed. In this case the control sieves corresponding to the cyclonesare suitably arranged in such way that the coarse, not finely groundproduct separated by the sieves by gravity can be recycled to the feedhopper 23.

In the above example grinding of ore in grate mills has been described,since same are the most effective ones in wet grinding. In certaincases, for instance if the material tends to clog the grates, traversetype mills are to be preferred. In such case the ground productdischarged from the mill will be finer than in grinding in grate millsso that there is less advantage in using an air lift. Therefore, in suchcases it may be suitable to substitute a rotary pump for the air liftfeeding the cyclone 42 directly. The operation of said pump is thencontrolled in an analogous manner as described for the pump 58 connectedto the second hydrocyclone, whereas the control of the opeartion of thecyclone, measuring of amounts and density of pulp are effected in asimilar way as described for air lift pumping.

Having now described the invention, what we claim as new and desire tosecure by Letters Patent, is:

1. A device for comminuting a material consisting of solid pieces ofpreferably crystalline structure, for instance ore, to a watersuspension of finely ground material having a substantiallypredetermined density, comprising a rotatable grinding drum providedwith feeding devices for material to be ground and devices fordischarging a pulp comprising finely and partly finely ground materialand also with a driving device, in which grinding drum the grindingmaterial without the presence of foreign grinding bodies is caused toact as grinding bodies and crush and grind itself, a pipe with a controlvalve for adjustably adding water to the grinding drum, devices forseparation of the pulp comprising finely or partly finely groundmaterial from coarse pieces of the material subjected to grinding,devices for conveying the pulp comprising finely and partly finelyground material discharged from the grinding drum, a flow meter formeasuring the rate of flow of the pulp, a density meter, means conveyingsaid pulp through said flow meter and said density meter, meanscontrolled by said flow meter and density meter to separate the finelyground material from the partly ground material to a predetermined ratioof separa tion independent of variations of the incoming quantity ofsolids and return the pulp comprising pantly finely ground material tothe grinding drum for furnishing a closed grinding circuit, and aregulating member which is adapted to be actuated by said density meterfor governing the control valve to increase the addition of water to thegrinding drum when measuring a pulp density above a predetermined valueand for reducing the addition of water to the grinding drum whenmeasuring a pulp density below said predetermined valve.

2. A device as claimed in claim 1, and means including said flow meterfor continuously measuring the quantity of solids per unit of timedischarged from the grinding drum, devices for comparison of said valuewith the quantity of grinding material fed into the grinding drum, andmeans controlled by said last-named means and lastnamed devices toregulate the number of revolutions per minute of the grinding millmotor.

3. A device for comminuting a material consisting of solid pieces ofpreferably crystalline structure, for instance ore, to a watersuspension of finely ground material having a substantiallypredetermined density, comprising a rotatable grinding drum providedwith feeding devices for material to be ground and devices fordischarging a pulp comprising finely and partly finely ground materialand also with a driving device, in which grinding drum the grindingmaterial without the presence of foreign grinding bodies is caused toact as grinding bodies and crush and grind itself, a pipe with a controlvalve for adjustably adding water to the grinding drum, a grate arrangedat the discharge end of the grinding drum with radial lifters forseparation of said pulp comprising finely and partly finely groundmaterial from coarse pieces of the material subjected to grinding,devices for conveying said pulp comprising finely and partly finelyground material discharged from the grinding drum, to two subsequenthydrocyclones arranged in series for repeated treatment of the pulpdischarged from the grinding drum to separate and discharge the pulpcomprising finely ground material and return the pulp comprising partlyfinely ground material to the grinding drum for furnishing a closedgrinding circuit, devices for regulating the bottom apertures of thecyclones, a device for continuously measuring the density and a devicefor measuring the quantity of solids in the suspension fed to therespective hydrocyclones, a device for measuring the density of finelyground pulp separated from the second hydrocyclone and a device adaptedto be actuated by signals from the last mentioned device for theoperation of a control valve, engaged to a water pipe connecting thesecond hydrocyclone for regulating the water supply to the secondhydrocyclone.

4. A device as claimed in claim 3, comprising a variable capacity pumparranged between the first hydrocyclone and the inlet of the secondhydrocyclone.

5. A device as claimed in claim 3, in which the means for passing thepulp comprising finely and partly finely material discharged from thegrinding drum to the hydrocyclones consists of an air lifit pumping thepulp to an equalizing container mounted above the first hydrocyclone andthat between said container and the first hydrocyclone placed under saidcontainer a downcomer is arranged to convey the pulp by gravity to saidfirst hydrocyclone.

References Cited in the file of this patent UNITED STATES PATENTS2,499,347 Adams Mar. 7, 1950 2,533,852 Tietig Dec. 12, 1950 2,534,656Bond Dec. 1-9, 1950 2,668,667 Fern et al Feb. 9, 1954 2,833,482 WestonMay 6, 1958 OTHER REFERENCES Hardinge: Making Rock Grind Itself, June1955, pages 84-90, Engineering and Mining Journal, volume 156, Number 6.

1. A DEVICE FOR COMMUNICATING A MATERIAL CONSISTING OF SOLID PIECES OFPREFERABLY CRYSTALLINE STRUCTURE, FOR INSTANCE ORE, TO A WATERSUSPENSION OF FINELY GROUND MATERIAL HAVING A SUBSTANTIALLYPREDETERMINED DENSITY, COMPRISING A ROTATABLE GRINDING DRUM PROVIDEDWITH FEEDING DEVICES FOR MATERIAL TO BE GROUND AND DEVICES FORDISHCARGING A PLUP COMPRISING FINELY AND PARTLY FINELY GROUND MATERIALAND ALSO WITH A DRIVING DEVICE, IN WHICH GRINDING DRUM THE GRINDINGMATERIAL WITHOUT THE PRESENCE OF FOREIGN GRINDING BODIES IS CAUSED TOACT AS GRINDING BODIES AND CRUSH AND GRIND ITSELF, A PIPE WITH A CONTROLVALVE FOR ADJUSTABLY ADDING WATER TO THE GRINDING DRUM, DEVICES FORSEPARATION OF THE PULP COMPRISING FINELY OR PARTLY FINELY GROUNDMATERIAL FROM COARSE PIECES OF THE MATERIAL SUBJACTED TO GRINDING,DEVICES FOR CONVEYING THE PULP COMPRISING FINELY AND PARTLY FINELYGROUND MATERIALS DISCHARGED FROM THE GRINDING DRUM, A FLOW METER FORMEASURING THE RATE OF FLOW OF THE PULP, A DENSITY METER, MEANS